Bacteriochlorins absorb strongly in the near-infrared spectral region1 and hence are attractive candidates for a wide variety of photochemical studies, including artificial photosynthesis,2-9 photodynamic therapy (PDT),10-23 optical imaging,24-26 and perhaps flow cytometry.24,27 Naturally occurring bacteriochlorophylls a, b, and g contain the bacteriochlorin chromophore and provide the basis for light-harvesting processes and electron-transfer reactions in bacterial photosynthesis (Chart 1, panel A).28 Bacteriochlorophylls also possess a five-membered ring (ring E) that encompasses the 13- and 15-positions; the ring contains a 131-oxo moiety and a 132-methoxycarbonyl substituent. Synthetic manipulation of bacteriochlorophylls has afforded a number of derivatives including (i) bacteriopyropheophorbides, which lack the 132-methoxycarbonyl substituent, the phytyl-like chain, and the central magnesium;2-4,29,30 and (ii) bacteriopurpurinimides (hereafter referred to as bacteriochlorin-imides), which bear a six-membered imide ring (Chart 1, panel B).6,12,16,17,31-37 
The presence of the imide ring in bacteriochlorin-imides provides a number of attractions including (1) a hyperchromic and bathochromic shift of the long-wavelength absorption band; (2) the ability to introduce diverse groups at the nitrogen of the imide ring;38 and (3) increased stability of the macrocycle toward routine handling due to the presence of the second carbonyl group at the 15-position. So far, bacteriochlorins bearing the five membered oxopentano or six-membered imide ring have only been available from the natural compounds or upon semisynthesis therefrom, respectively, although synthetic porphyrins and chlorins with a wide variety of annulated rings have been prepared.39,40 Two significant problems in the preparation of derivatives of bacteriochlorophylls include limited stability36,41,42 and poor synthetic malleability owing to the presence of a nearly full complement of substituents about the perimeter of the macrocycle.13,18 The synthesis of bacteriochlorins by reduction or addition of porphyrins or chlorins is appropriate for a number of applications but generally suffers from a lack of regiocontrol.43

Over the past decade we have been developing a de novo synthesis of bacteriochlorins.44-46 The route affords bacteriochlorins wherein each pyrroline ring contains a geminal dimethyl group rather than the trans-dialkyl and exo-ethylidene moieties of the naturally occurring bacteriochlorophylls. The geminal dimethyl group has the attractive feature of stabilizing the macrocycle toward adventitious dehydrogenation. Synthetic bacteriochlorins bearing diverse substituents at specific sites in the pyrrolic units have been prepared, and selected derivatization processes of the bacteriochlorins have been examined (including regioselective bromination); however, no annulated rings have yet been introduced.44-51 